1. Field of the Invention
This invention relates to a method, a terminal and a system for communication and, more particularly, to a method, a terminal and a system suitable for multi-station video communication.
2. Description of the Related Art
Recently, image compression coding techniques have been developed remarkably and digital communication networks have widely been extended. Audio and video service rules and protocols and multi-media multiplexing frame construction prescriptions for TV telephone communication or TV conferences have been provided as recommendations, while TV telephone apparatuses, TV conferences systems and various audio and video terminals have been proposed. In particular, a multi-station TV conferences system in which terminals in at least three stations are connected for TV conferences or video conferences has attracted attention.
A multi-station TV conference system, such as that shown in FIG. 10, is known which uses a multi-station communication control unit (hereinafter referred to as "MCU") for mutual connection between TV conference terminals in many stations to realize multi-station TV conferences.
Another system has been proposed in which TV conference terminals are connected by a circuit in the form of a loop or a bus and predetermined data is relayed by the terminals, thereby realizing multi-station TV conferences. More specifically, various kinds of data including video data and set in data fields assigned to the terminals are circulated through the circuit while being generated or updated by each terminal as desired.
The former multi-station TV conference system, in which all the TV conference terminals are connected to the MCU, and in which all the TV conference terminals and communications between the terminals are monitored and controlled by the MCU, entails a drawback described below. First, a TV conference cannot be held if the MCU is unable to operate or is being used for another multi-station TV conference. Second, because the MCU requires a plurality of communication control, encoding and decoding circuits to connect a plurality of terminals, it is very high-priced. Third, the number of conferees is limited by the terminal (line) capacity of the MCU.
The latter system, in which TV conference terminals are connected by a circuit in the form of a loop or a bus through which data having a multi-frame structure in which data fields provided for the data terminals are allotted to a multiplicity of frames is relayed or updated by the terminals while being circulated through the circuit, has the advantage of requiring no MCU. However, since the entire data is circulated through the communication circuit, each terminal receives the data that it has generated. It is meaningless for each terminal to receive the data that it has inserted in the communication circuit. In other words, a part of the communication circuit is always occupied essentially meaninglessly by some data. In the case of communication using moving picture information, the amount of transmitted information is so large that reductions in the communication speed and the efficiency of use of the communication circuit due to such unnecessary occupation of the communication circuit are not negligible. In such a case, the amount of useful information to be transmitted may be limited.